1. Field of the Invention
This invention relates generally to a single spatial mode laser and, more particularly, to a high power, single-mode, diode-pumped, fiber ribbon laser having a rectangular shaped core, where laser waveguides of the laser employ mode filters to remove higher-order propagation modes from the core.
2. Discussion of the Related Art
There exists a need in the art for an electrically driven laser that has a high average power, but does not employ chemical laser fuel or effluent. These types of lasers have many applications, including military applications against a variety of airborne threats, such as ballistic missiles, cruise missiles, and tactical aircraft. Diode-pumped, solid-state lasers employing an array of fiber laser waveguides or amplifiers is one known laser that satisfies this need. A practical technique is needed to make fiber lasers into rugged devices without complex optical components to produce useful and affordable laser systems.
Typically, for applications of this type, the laser system must employ a fiber or core that generates a single-mode laser beam. This is because a single-mode laser beam generates the most intensity or power per unit area when the beam is focused. As the number of transverse modes of the laser beam increases, the size of the beam spot that can be focussed also increases as a result of beam diffraction. This reduces the beam power per unit area, which reduces its intensity.
Diode-pumped, dual-clad ytterbium-doped glass fiber lasers have been used in the art to generate single-mode laser beams, generally having an output power up to 50 watts. The fibers in these lasers typically employ a round core having a diameter on the order of 5-8 microns to generate the single-mode laser beam. An inner cladding layer around the core traps the single-mode beam within the core, and an outer cladding layer reflects pump light across the core to be absorbed. A discussion of this type of fiber laser can be found in the proceedings for a conference on Advanced Solid-State Lasers, including DiGiovanni, David J., "High Power Fiber Lasers and Amplifiers," Fibers and Waveguide Devices, Feb. 3, 1999, pgs. 282-284, and Nilsson, J. et al., "Widely tunable high-power diode-pumped double-clad Yb.sup.3+ -doped fiber laser," Fibers and Waveguide Devices, Feb. 3, 1999, pgs. 285-287.
It is desirable in the art to increase the power output of single-mode fiber lasers for certain applications. The power output of the laser can be increased by increasing the length of the core and providing more optical pump light. However, the material of the core has power limits that if exceeded may damage the core material. More optical power can also be provided by making the core diameter larger. However, as the core diameter increases, the generation of higher-order modes begin to develop, and it becomes increasingly more difficult to limit the beam to a single-mode. Once a certain core size is reached, it is virtually impossible to limit the propagation modes to a single-mode. Further, as the size of the core and the power increases, the generation of heat in the core also increases. Cooling systems are known to reduce this heat, but larger diameter cores makes it more difficult to remove the heat from the center of the core. Therefore, a heat gradient may exist across the core, which causes the laser to decrease its performance.
The core of a dual-clad fiber laser tends to have a length on the order of 50 m. Therefore, the core is typically wrapped around a mandrel to decrease the size of the laser system and give the core structural rigidity. As the size of the core and associated cladding increases, it is more difficult to bend the core around the mandrel and still maintain mode-control, thus causing the mandrel to have to be larger. Also, the size of the core determines how tightly the core can be wrapped on the mandrel before stresses reduce or eliminate mode control caused by light leakage.
What is needed is a single-mode fiber laser that has increased power over those fiber lasers known in the art without losing mode control and that allows effective laser cooling. It is therefore an object of the present invention to provide such a laser.